media assets: add file_key, is_public columns.

`file_key` is a random 9-character base-62 string that will be used as
the image filename in the future.

`is_public` is whether the image can be viewed without authentication or not.

Users running downstream boorus must run `bin/rails db:migrate` and
`script/fixes/109_generate_media_asset_file_keys.rb` after this commit.

db/migrate/20220504235329_add_file_key_to_media_assets.rb

@@ -0,0 +1,9 @@
+class AddFileKeyToMediaAssets < ActiveRecord::Migration[7.0]
+  def change
+    add_column :media_assets, :file_key, :string, null: true
+    add_column :media_assets, :is_public, :boolean, default: true, null: false
+
+    add_index :media_assets, :file_key, unique: true
+    add_index :media_assets, :is_public, where: "is_public = FALSE"
+  end
+end